To avoid this confusion, let's elaborate the main difference between
User Profile Service and
User Profile Synchronization service.
User Profile Service:
This service is used to import the users from AD it's one-way sync direction from AD to SharePoint, So if you update any user information from SharePoint it will not be synced to AD.
User Profile Synchronization service:
This service is two-way sync direction from AD to Sharepoint and vise verse. it provides a reliable way to synchronize groups and user profile information that is stored in the SharePoint Server 2013 profile database together with information that is stored in directory services across the enterprise.
So in your case, the users have been imported from AD to SharePoint because the User Profile Service is started and running.
For more details check Overview of the User Profile service application in SharePoint Server 2010